This invention relates to magnetic tape drives, and more particularly to apparatus for sensing the position of a magnetic tape in the vacuum columns of a drive. Basically, vacuum columns are included in a tape drive to allow the tape to be accelerated at a rate faster than the acceleration of the tape reels. By providing a vacuum column between each tape reel and a read/write head, tape may be moved in or out of the columns until the reel velocity equals the tape velocity across the head.
In order for such a system to operate properly, it is necessary to sense the position of the tape in the vacuum columns. Otherwise, the tape could get completely pulled out of one of the vacuum columns and tear. Alternatively, the tape could completely fill one of the vacuum columns which also would cause a malfunction. These problems are overcome by sensing the position of the tape in the columns and by accelerating the tape reel in response thereto.
A variety of tape position sensing apparatus has been described in the prior art. One such apparatus that is currently widely used operates on a capacitive sensing principal. Basically, in this apparatus, movement of the tape in the vacuum columns causes a corresponding movement between two plates of a variable capacitor. And by electronically sensing this capacitance, the tape position is determined.
One problem however, with this prior apparatus is that a time lag exists between the instantaneous position of the tape in the vacuum column and the corresponding instantaneous capacitance of the variable capacitor. Primarily, this time lag is caused by air bubbles getting trapped between the plates of the variable capacitor as they are moved together. As a result, the capacitance of the variable capacitor is less than it should be. These air bubbles typically take several milliseconds to escape from between the capacitor plates. Thus, the instantaneous position of the tape in the vacuum cannot be determined accurately from the instantaneous capacitance reading. This of course, limits the tape speed, and the rate at which the data can be read from or written thereto.
Accordingly, it is one object of the invention to provide a magnetic tape having an improved tape position sensing apparatus.
Another object of the invention is to provide an improved variable capacitor suitable for sensing the position of a magnetic tape in a vacuum column.
Still another object of the invention is to provide a magnetic tape position sensing apparatus that is essentially free from capacitance variations caused by air bubbles being temporarily trapped between the capacitive plate.